Lasers and other optical signal sources are well established as the preferred transmission source for many communication and optical sensing applications. To facilitate uniformity in the transmission of information signals via optical mediums, the International Telecommunications Union (ITU) has specified a standard grid of frequency channels which are separated by multiples of 50 or 100 GHz, or 400 to 800 pm in wavelength. These standards have since been used to develop systems which utilize dense wavelength division multiplexing (DWDM) to transmit multiple channels, often with significantly less than 100 GHz of channel separation, over a single fiber optic medium. Due to these narrow channel separations, an optical source, such as a laser, must operate reliably on a single longitudinal mode and be controlled in frequency to within a few GHz of a given channel's center frequency. As the demand for bandwidth increases, it is anticipated that channel spacing of 50 GHz or even 25 GHz will become desirable by owners and operators of fiber optic communication systems. Such ever narrower channel spacing will further accentuate the need for lasers to tightly control the mode and frequency of an output laser beam.
Commonly, distributed feedback lasers (DFBs) are used to generate the laser beam (or light) used for each communications channel supported by a given communications system. DFBs consist of a monolithic resonator structure with a distributed Bragg reflector integrated into the semiconductor laser waveguide structure. However, DFBs only operate on a single frequency and are not tunable. For example, if 80 channels are utilized in a given communications system, then 80 DFBs are generally needed (i.e., one for each channel). As is readily apparent, supporting and providing 80 lasers (often on each of a multitude of nodes in a communications system) is cumbersome, expensive and presents numerous supply and logistical issues. Further, as the channel spacing continues to decrease, the number of lasers desired for a given transmission medium, using DFBs, could become quite large. Thus, there is a need for tunable lasers that can be reliably tuned, locked and stabilized at a given frequency, without requiring extensive delay or the need to utilize additional calibration equipment, such as spectrometers.
Various types of tunable lasers are currently available. These include vertical cavity semiconductor lasers (VCSEL), segmented-grating distributed Bragg reflector lasers (SGDBR) and external cavity semiconductor lasers (ECLs). As is discussed in greater detail in the '408 application, each of these types of tunable lasers have advantages and disadvantages. The advantages and disadvantages of VCSELs and SDCBRs, when compared with ECLs, have led many developers of tunable lasers to direct their attention to ECLs.
Various implementations of ECLs currently exist. Some of these implementations are described in P. Zorabedian, “Tunable External Cavity Semiconductor Lasers,” in Tunable Laser Handbook (F. J. Duarte Ed.), Academic Press, San Diego, 1995. Additional ECL designs can also be found, for example, in U.S. Pat. No. 5,771,252, entitled “External Cavity, Continuously Tunable Wavelength Source,” and in co-pending U.S. patent application Ser. No. 09/728,212, filed Nov. 29, 2000, in the name of John H. Jerman, et al. and entitled “Tunable Laser with Microactuator,” (hereinafter, the “'212 application”) the entire contents of each of which are incorporated herein by reference.
In general, ECLs, like most tunable lasers, utilize an optical filter in a resonator cavity to ensure that losses are large for all but the target frequency and for one of the many modes that overlap the gain curve for such target frequency. As is common to all lasers, and true for ECLs in particular, it is possible to adjust the absolute frequencies of a plurality of modes by changing the length of the resonator cavity. Further, ECLs and other tunable lasers commonly include a mechanism by which the frequency of the optical filter may be changed (within a predefined range). Various approaches for controlling the frequency of ECLs have been proposed. For example, co-pending U.S. patent application Ser. No. 10/099,412, filed Mar. 15, 2002, in the name of inventors Jill D. Berger, et al. and entitled “Apparatus for Frequency Tuning and Locking and Method for Operating Same,” (hereinafter, the “Frequency Tuning Application”), the entire contents of which are incorporated herein by reference, describes various embodiments of wavelength lockers (WLL) that may be utilized to tune an ECL to a target frequency.
While WLLs and similar apparatus have shown to be effective in tuning an optical filter to a target frequency, frequency control is merely the first step in tuning a laser. As mentioned previously, the mode at which the laser operates must also be properly tuned whenever the optical filter's frequency is changed. It has been the combination of these two steps, efficiently tuning the frequency while stabilizing the mode, which have presented obstacles to the cost-effective implementation of ECLs and other tunable lasers. More specifically, controlling the mode while changing the frequency, and vice versa, has proven to be extremely problematic because as a given laser is tuned to a target frequency, a mode hop may often undesirably occur. Tuning a laser can be further exacerbated by the fact that many tunable lasers utilize multiple optical filters, each of which must be correctly tuned in order to provide an output laser beam at a target frequency and mode. Multiple filters are commonly used in the before mentioned ECL designs described by Zorabedian and also for many segmented-grating distributed Bragg reflector lasers (SGDBR), such as those described in L. A. Coldren and S. W. Corzine's Diode Lasers and Photonic Integrated Circuits, Wiley, N.Y., 1995 and also in U.S. Pat. No. 4,896,325, which is entitled “Multi-Section Tunable Laser with Differing Multi-Element Mirrors,” the entire contents of each of which are incorporated herein by reference.
One approach for controlling the mode of a tunable laser utilizes encoders, which measure the precise position of actuators used to control the various elements utilized in a given optical filter. For example, an external cavity filter often utilizes at least one actuator such as a micro-electro-mechanical-system (MEMS) actuator. Since MEMS actuators exhibit hysteresis and resonant behavior, encoders may be utilized to determine the precise relative position of the mirror. However, encoders are undesirable in some tunable laser applications because of the size and the inherent difficulty in precisely calibrating encoders. Therefore, there is a need for a tunable laser for which the mode may be stabilized while the frequency is efficiently and reliably tuned to a desired target frequency.